1. Field of the Invention
The present invention relates to a method of measuring a leveling plane suited for checking the current state of a leveling plane which is used in an auto-leveling mechanism provided in a projection exposure apparatus which is employed for manufacturing semiconductor elements, liquid crystal display elements, thin film magnetic heads or the like in a lithographic process.
2. Related Background Art
For manufacture of semiconductor elements, liquid crystal display elements, thin film magnetic heads or the like in a lithographic process, a projection exposure apparatus is used which exposes pattern images of photomasks or reticles (hereinafter referred to only as "reticles") on photosensitive substrates through a projection optical system. In order to expose the pattern image of a reticle at high resolution on a photosensitive substrate in such a projection exposure apparatus, it is necessary to perform exposure by matching the exposure surface of the photosensitive substrate with the best image forming plane of the projection optical system within the range of the focal depth. As disclosed in U.S. Pat. No. 4,558,949 for example, a conventional projection exposure apparatus for performing this exposure comprises an auto-leveling mechanism for keeping the exposure face of a photosensitive substrate in parallel with the best image forming plane of the projection optical system, and an auto-focussing mechanism for keeping the position of the exposure face of the photosensitive substrate along the optical axis (i.e., the focal position) at the best focal position which is the best image forming position of the exposure face. In the following description, semiconductor wafers coated with photoresists or the like will be used as photosensitive substrates.
With the conventional leveling mechanism, the surface of a wafer mounted on a leveling stage by a wafer holder is selected as a leveling plane, and the amount of inclination of the leveling plane with respect to the best image forming plane of the projection optical system is obtained as will be described later. The leveling plane is held in an inclined state by the amount of inclination. A leveling sensor is provided which detects the average value of Inclination of a surface in a predetermined test area in the exposure field of the projection optical system. An output signal which is to be supplied by the leveling sensor indicating the state in which the surface of a wafer in the test area is in parallel with the best image forming plane is previously obtained. Thereafter, the inclination of the wafer is automatically controlled by a leveling stage so that the leveling sensor outputs the signal. In other words, auto-leveling is performed.
Conventionally, the amounts of inclination of the leveling plane and the best image forming plane are obtained from a "stage running plane" of an XY stage, as a datum (reference) plane, for positioning a wafer in a plane substantially perpendicular to the optical axis of the projection optical system (the plane being called an XY plane). The stage running plane is defined by a plane formed by a locus of an arbitrary point on the surface of a wafer mounted on the XY stage by a leveling stage and a wafer holder and the like, when the XY stage moves in the XY plane.
The amounts of inclination of the best image forming plane and the leveling plane are measured on the basis of the stage running plane, respectively. The difference between them is obtained as an inclination error of the leveling plane with respect to the best image forming plane. The projection exposure apparatus is provided with a Z stage for positioning the wafer along the optical axis (in the Z direction), and the auto-focus mechanism has a focal position detecting device for measuring focal positions on the surface of the wafer corresponding to the predetermined measuring points in the exposure field of the projection optical system.
In order to measure the inclination of the best image forming plane with respect to the stage running plane, the XY stage is operated so as to move measuring points on the surface of the wafer to a first projection point in the exposure field of the optical system, and a mark image for measuring the focal position of a reticle is projected on the projection point, after the leveling stage has been locked in a predetermined inclined state and the focal positions of the measuring points on the surface of the wafer have been set at a first focal position by means of the focal position detecting device. Focus measuring mask images are projected by gradually changing the focal position of the first position point, and the best focal position for the first projection point is obtained by measuring the size of the mark images after development.
Similarly, the focal positions of other projection points in the exposure field of the projection optical system are changed and their best focal positions are obtained by exposing focus measuring mark images. The flatness of the wafer surface is corrected by continuously joining the obtained focal positions whereby the inclination of the leveling plan with respect to the stage running plane is obtained.
If, in the conventional method, the wafer surface in the detecting area of the leveling sensor is irregular, the signals detected by the leveling sensor change according to the irregularity and measuring errors occur, when a plurality of the measuring points on the wafer are moved to the measuring point of the focal position detecting device by moving the XY stage and then the flatness of the wafer is measured in order to measure the inclination of the leveling plane with respect to the stage running plane. Thus, so-called super-flat wafers which have extremely good flatness are required for performing the conventional method.
When such super-flat wafers cannot be obtained, it is necessary to find portions of normal wafers which exhibits good flatness. However, such measurement requires a long time. Even if wafers are substantially super-flat, their surface still has very small depressions and projections which exhibit irregularity. Such irregularity on the wafer surface produces a measurement error, causing difficulty in measurement at a high accuracy.
The inclination of the leveling plane with respect to the stage running plane is found indirectly in steps by obtaining the amount of inclination of the best image forming plane with respect to the stage running plane and the amount of the inclination of leveling plane with respect to the stage running plane. Thus, the measuring process is complicated and it takes a long time to perform measurement.